This invention relates to techniques for routing optical wavelengths, and more particularly relates to wavelength-selective optical routing and switching techniques.
The increasing use of all-optical fiber networks as backbones for global communication systems has been based in large part on the extremely wide optical transmission bandwidth provided by optical fiber. This has accordingly led to an increased demand for the practical utilization of the full optical fiber bandwidth available, to, for example, increase communication system user capacity. In the prevailing manner for exploiting optical fiber bandwidth, wavelength-division multiplexing (WDM) and wavelength-division demultiplexing (WDD) techniques are employed to enable the simultaneous transmission of multiple independent optical data streams, each of a distinct wavelength, on a single optical fiber, with wavelength-selective WDM and WDD control provided for coupling of the multiple data streams with the optical fiber on a wavelength-specific basis. With this capability, a single optical fiber can be configured to simultaneously transmit several optical data streams, e.g., ten optical data streams, that each might not exceed, say, 10 Gb/s, but that together represent an aggregate optical fiber transmission bandwidth of more than, say, 100 Gb/s.
In order to increase the aggregate transmission bandwidth of an optical fiber, it is generally preferred that the spacing of simultaneously transmitted optical data streams, or optical data "channels," be closely packed, to accommodate a larger number of channels. In other words, the difference in wavelength between two adjacent channels is preferably minimized. Current transmission standards require accommodation of 100 GHz, corresponding to 0.8 nm, separation between adjacent channels, and in some applications, channel separation is preferably reduced to 50 GHz.
This desire for closely-spaced optical transmission channels results in the need for fine wavelength resolution and thereby complicates the wavelength-selective WDM and WDD operations required for simultaneous transmission of the channels. Historically, the well-known Dragone filter design has been employed to perform wavelength-selective WDM and WDD operations with a reasonable degree of wavelength resolution. The Dragone filter functions in the manner of a discrete prism to resolve all transmission channel wavelengths incident at its input into spatially separated transmission channels. Once spatially separated, one or more of the transmission channels can then be extracted, or dropped, from the set of channels. Two Dragone filters can be configured in back-to-back fashion to enable resolution of all transmission channels, dropping of a selected one or more channels, and then injection of substitute channels and recombination for simultaneous transmission.
The use of Dragone filters in this manner for wavelength-specific, single channel WDM and WDD operations is problematic in that the size of the filter structure, which is microfabricated, typically exceeds about 2 cm.sup.2, a size that is increasingly becoming hard to accommodate as overall optical transmission system componentry size shrinks in response to cost considerations. Inter-channel crosstalk can also occur during the resolution and recombination operations, resulting in echoes of a removed channel at the recombined output.
Use of a Dragone filter for wavelength-specific, single channel operations at, e.g., a wavelength-specific user node on an optical communications network, is inefficient in that all copropagating channels must be spatially separated to extract only the single channel of interest to the node. As new wavelengths are added to the communications network, the Dragone filters employed at each of the nodes must accordingly be replaced to accommodate spatial separation of added wavelengths. These limitations of the Dragone filter generally characterize the insufficiency of conventional all-optical wavelength-selective routing techniques in meeting the increasingly complex requirements of optical networks.